A testing input that generates an unbounded PCollection of elements, advancing the
watermark and processing time as elements are emitted. After all of the specified elements are
emitted, ceases to produce output.

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Composite transforms, which are defined in terms of other transforms,
should return the output of one of the composed transforms. Non-composite
transforms, which do not apply any transforms internally, should return
a new unbound output and register evaluators (via backend-specific
registration methods).